A Winchester 21 with 26" or 28" barrels would be about perfect. As Kevin mentioned, 20 or 28 ga tubes would be great. I'd prefer 28 gauge personally. My experience there led to the belief that full choked long barrel guns were a detriment. Most shots were fairly close and fast. Whatever you decide, please do not make the mistake of taking a repeater. Hard for us in the states to comprehend just how painful it becomes to shove hundreds of rounds into a magazine. Also how easy it is for those mechanical mousetraps to stop functioning. My first day gun was a near new 20 gauge Benelli and it proved to be a piece of junk. Borrowed a 28 ga Beretta O/U after and it proved to be about perfect.
I'll probably never go again but if I did and if I won the lottery it'd be nice to have a pair of doubles. Teach the bird boy how to load and work with you.
Actually, my problem was that after about 500 rounds I'd get bored and want to do something else. So I'd pack up and go watch other shooters--lots of fun to verbally critique their shooting. Couple times I thought Mike was gonna let a round loose in my direction.
Oh, 16 gauge ammunition. It was my understanding that there are a lot of 16's in use by locals and fodder is readily available for the asking. Most lodges don't have it in their storeroom cause it's seldom called for. A small store down the road from Los Charnares in the town of Santa Fe (?) had all the 16's Mike could use.